In sheet-fed offset printing machines, a printing plate having a leading edge--a print start--and a trailing edge--a print end--is typically fastened on the plate cylinder by means of tension rails arranged in a cylinder channel. These tension rails are assigned respectively to the print start and the print end. In order to attach a printing plate, the leading edge (print start) is first inserted and clamped into the corresponding tension rail. Next, the trailing edge (print end) is similarly clamped into a second tension rail. Finally, the printing plate is tensioned by the exertion of force. The accurate and precise introduction of the trailing edge of a new printing plate can be very time consuming under certain circumstances. In particular, the trailing edge must be precisely aligned when large format printing is involved. Thus, the design of the print-end tension rail is of great importance in reducing the time spent in drawing off an old printing plate and drawing on a new plate.
DE 3,940,795 C2, DE 3,940,796 C2 and EP 0,431,575 A2 disclose systems capable of performing automatic printing plate changes. In these systems, the drawing off of an old printing plate from the plate cylinder and the drawing on of a new printing plate onto the latter take place automatically. In addition to storage and reception regions for the printing plates, these systems also have transport devices for feeding a new printing plate and conveying an old printing plate away while the plate cylinder is being rotated in the appropriate direction. The printing plates are fastened onto the plate cylinder by means of tension rails which, as indicated in each of the individual publications, have remotely actuatable devices for corresponding clamping or tensioning.
EP 0,431,575 A2 discloses the use of printing plates having a bevel at their trailing edge (print end) which can be inserted into a radially oriented gap between two clamping pieces. After the printing plates are positioned, the two clamping pieces are pivoted in a virtually azimuthal direction by an eccentric shaft in order to achieve clamping, and ultimately, tensioning. This approach has many disadvantages. First of all, the new printing plates must be bevelled at their trailing edges by a special device. This bevelling must be carried out very carefully because possible corrugations in the bevelled region can lead to tilting and therefore to problems when introducing the plates into the gap of the tension rail. Therefore, the approach presents difficulties in manufacturing printing plates. Further, in view of the problems presented by corrugations, it seems scarcely possible to supply used printing plates for a system of this kind. Moreover, a bevelled print end cannot be conveyed directly by a transport system having a roller nip.
Bevelled printing plates cannot be used in the printing plate changing systems disclosed in DE 3,940,793 C2 and DE 3,940,796 C2 because of the specially designed print-end tension rails. The clamping flaps causing gripping have a specially arranged pivoting pole which is disclosed in DE 3,626,936 C1. These clamping flaps make it possible to place the trailing edge (print end) of the printing plate onto a lower clamping device. However, this procedure entails a relatively large pivoting angle of the clamping flap. A disadvantage of this large pivoting angle is that a clamping flap which is pivoted into the opening position projects correspondingly far from the reference-circle circumference of the print cylinder and thus impedes a pressure roller which causes the printing plate to be laid down.
German Utility Model 6,491,597 discloses a tension rail for use in a plate cylinder in a printing machine wherein the tension rail is assigned to the trailing edge or print end of a printing plate. This tension rail consists of two clamping pieces which are movable relative to one another for gripping the trailing edge of the printing plate. Further, in order to enable tensioning of the printing plate, the tension rail is pivotably mounted about an axis extending parallel to the plate cylinder. A disadvantage of the device described is that interaction with automatic plate changing systems is impossible.
U.S. Pat. No. 1,521,665, U.S. Pat. No. 3,107,609 and EP 0,458,323 A1 teach the tensioning of a printing plate with the force of compression springs. The printing plates can be released via a corresponding exertion of force counter to the spring forces.